Nonaqueous secondary battery and method of using the same

ABSTRACT

A nonaqueous secondary battery having a positive electrode having a positive electrode mixture layer, a negative electrode, and a nonaqueous electrolyte, in which the positive electrode contains, as an active material, a lithium-containing transition metal oxide containing a metal element selected from the group consisting of Mg, Ti, Zr, Ge, Nb, Al and Sn, the positive electrode mixture layer has a density of 3.5 g/cm 3  or larger, and the nonaqueous electrolyte contains a compound having two or more nitrile groups in the molecule

This application is a Divisional of co-pending application Ser. No.11/976,566, filed on Oct. 25, 2007, the entire contents of which arehereby incorporated by reference into the present application and forwhich priority is claimed under 35 U.S.C. §120; and wherein co-pendingapplication Ser. No. 11/976,566 claims priority under the provisions of35 U.S.C. §119 to Japanese Application Serial No. 2006-290637 filed onOct. 26, 2006, the entire contents of which are hereby incorporated byreference into the present application.

FILED OF THE INVENTION

The present invention relates to a nonaqueous secondary battery having ahigh capacity, good charge-discharge cycle characteristics and highreliability such as safety.

RELATED ART

In recent years, the secondary battery is an indispensable, importantdevice as a power source of a personal computer or a cellular phone, ora power source for an electric vehicle or an electric power storage.

In particular, in applications for a mobile communication device such asa portable computer and a personal digital assistant, the battery isrequired to be made smaller and to trim weight. Under the currentcircumstances, however, the system of the battery is not easily madecompact or lightweight, since an electric power consumed by a back lightof a liquid crystal display panel or consumed to control the drawing ofgraphics is large, or the capacity of a secondary battery is notsufficiently large. In particular, a personal computer is progressivelymulti-functionalized by mounting a digital versatile disc (DVD) driveand so on. Thus, the power consumption thereof tends to increase. Forthis reason, it is highly required to increase the electric capacity ofa secondary battery, in particular, the discharge capacity, when thevoltage of a single battery is 3.3 V or higher.

Attention is paid to electric vehicles, which discharge no exhaust gasand make less noise in association with the increase of globalenvironmental problems. Recently, parallel hybrid electric vehicles(HEV), which adopt a system of storing regenerative energy generated atthe time of braking in a battery and making effective use of the energy,or using an electric energy stored in a battery at the time of enginestarting to increase the efficiency of the engine system, have gainedpopularity. However, since the electric capacity of the currently usedbattery is small, a plurality of batteries should be used to generate asufficient voltage. For this reason, problems such that a space in thevehicle should be made smaller and that the stability of the vehiclebody deteriorates arise.

Among secondary batteries, a lithium secondary battery using anonaqueous electrolyte attracts attention, since it generates a highvoltage, has a lightweight and is expected to achieve a high energydensity. In particular, a lithium secondary battery disclosed inJP-A-55-136131, in which a lithium-containing transition metal oxide,for example, LiCoO₂, is used as a positive electrode active material,and metal lithium is used as a negative electrode active material, isexpected to attain a high energy density, since it has an electromotiveforce of 4 V or higher.

However, at present, in the case of a LiCoO₂ based secondary batterywhich uses LiCoO₂ as a positive electrode active material and acarbonaceous material such as graphite as a negative electrode activematerial, a charge final voltage thereof is usually 4.2 V or less.According to this charging condition, the charge capacity is only about60% of the theoretical capacity of LiCoO₂. The electric capacity may beincreased by increasing the charge final voltage to higher than 4.2 V.However, with the increase of the charge capacity, the crystallinestructure of LiCoO₂ decays so that the charge-discharge cycle life maybe shortened, or the crystalline structure of LiCoO₂ may bedestabilized. Accordingly, the thermal stability of the batterydeteriorates.

To solve such a problem, many attempts have been made to add a differentmetal element to LiCoO₂ (cf. JP-A-4-171659, JP-A-3-201368, JP-A-7-176302and JP-A-2001-167763).

In addition, attempts have been made to use a battery in a high-voltagerange of 4.2 V or higher (cf. JP-A-2004-296098, JP-A-2001-176511 andJP-A-2002-270238).

In years to come, a secondary battery will be required to have a highercapacity and also better reliability than the conventional batteries. Ingeneral, the battery capacity can be greatly improved by raising thecontent of an active material in electrodes or by increasing anelectrode density, in particular, the density of a positive electrodemixture layer. However, according to such capacity-increasing measures,the reliability of the battery including storage characteristicsgradually decreases.

Accordingly, in order to meet requirements for the increase of theelectric capacity, it is highly desired to provide a battery which usesa material that generates a higher electromotive force (voltage range)than LiCoO₂ and has a stable crystalline structure capable of beingstably and reversibly charged and discharged, and which furthersatisfies reliability such that the safety of the conventional batteriescan be maintained and the battery does not expand during storage whenthe density of the positive electrode mixture layer is increased.

When the discharge final voltage of a conventional battery comprisingLiCoO₂ as a positive electrode active material is made higher than 3.2V, the battery cannot be completely discharged since the voltage in thefinal stage of the discharge significantly falls. Thus, an electricquantity efficiency of discharge relative to charging remarkablydecreases. Since the complete discharge cannot be attained, thecrystalline structure of LiCoO₂ easily decays, and thus thecharge-discharge cycle life is shortened. This phenomenon remarkablyappears in the above-mentioned high voltage range.

Under a charging condition that the final voltage at full charging isset to 4.2 V or higher in the conventional battery, apart fromshortening of the charge-discharge cycle life or the decrease of thethermal stability caused by the decay of the crystalline structure ofthe positive electrode active material, the electrolytic solution (asolvent) is oxidatively decomposed due to the increase of the activesites in the positive electrode active material, whereby a passivationfilm is formed on the surface of the positive electrode and thus theinternal resistance of the battery increases so that the loadcharacteristic may deteriorate.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nonaqueous secondarybattery having a high capacity, good charge-discharge cyclecharacteristics and high storage characteristics.

Accordingly, the present invention provides a nonaqueous secondarybattery comprising: a positive electrode having a positive electrodemixture layer, a negative electrode, and a nonaqueous electrolyte,wherein the positive electrode contains, as an active material, at leastone lithium-containing transition metal oxide comprising at least onemetal element selected from the group consisting of Mg, Ti, Zr, Ge, Nb,AI and Sn, the positive electrode mixture layer has a density of atleast 3.5 g/cm³, and the nonaqueous electrolyte contains a compoundhaving at least two nitrile groups in the molecule.

The nonaqueous secondary battery of the present invention can be chargedat a high voltage and thus has a large capacity, since the positiveelectrode mixture layer has a density of a specific value or larger soas to increase the filled amount of the positive electrode activematerial in the positive electrode mixture layer, and alithium-containing transition metal oxide comprising a specific metalelement, which is highly stable in a charged state at a high voltage, isused as a positive electrode active material.

Furthermore, since the positive electrode active material used in thenonaqueous secondary battery of the present invention has goodstability, the decay of the active material is suppressed when thecharging and discharging of the battery are repeated. Thereby, thenonaqueous secondary battery has good charge-discharge cyclecharacteristics.

In addition, the nonaqueous electrolyte used in the nonaqueous secondarybattery of the present invention contains a compound having at least twonitrile groups in the molecule. Such a nitrile compound acts on thesurface of the positive electrode and functions to prevent the directcontact of the positive electrode and the nonaqueous electrolyte. Thus,the reaction of the positive electrode and the nonaqueous electrolyte issuppressed, whereby the generation of a gas in the battery by such areaction is prevented. Accordingly, the suppression of the reaction ofthe positive electrode and the nonaqueous electrolyte and the use of thepositive electrode active material having high stability synergisticallyfunction to prevent the expansion of the battery during the storage ofthe charged battery at a high temperature and thus to improve thestorage characteristics of the battery.

Consequently, the nonaqueous secondary battery of the present inventionhas good charge-discharge cycle characteristics and good storagecharacteristics. The nonaqueous secondary battery of the presentinvention can be charged at a high positive electrode voltage in a rangeof 4.35 to 4.6 V with reference to the potential of lithium, and thus itcan be used in applications which require a high output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B schematically show one example of the nonaqueoussecondary battery of the present invention. FIG. 1A is a plan viewthereof and FIG. 1B is a partial vertical section thereof.

FIG. 2 shows a perspective view of the nonaqueous secondary batteryillustrated in FIGS. 1A and 1B.

DETAILED DESCRIPTION OF THE INVENTION

The nonaqueous secondary battery of the present invention may have sucha configuration that a laminate electrode body having a positiveelectrode with a positive electrode mixture layer and a negativeelectrode which are laminated each other with inserting a separatorbetween them, or a wound electrode body in which the laminate electrodebody is wound is enclosed within a housing together with a nonaqueouselectrolyte.

In the nonaqueous secondary battery of the present invention, thenonaqueous electrolyte is preferably a nonaqueous solvent-baseelectrolytic solution comprising an electrolyte salt such as a lithiumsalt dissolved in a nonaqueous solvent such as an organic solvent, fromthe viewpoint of electric characteristics or handling easiness. Apolymer electrolyte or a gel electrolyte may be used without anyproblem.

The solvent in the nonaqueous electrolytic solution is not particularlylimited, and examples thereof include acyclic esters such as dimethylcarbonate, diethyl carbonate, ethylmethyl carbonate, and methylpropylcarbonate; cyclic esters having a high dielectric constant, such asethylene carbonate, propylene carbonate, butylene carbonate, andvinylene carbonate; and mixed solvents comprising an acyclic ester and acyclic ester. Mixed solvents each comprising an acyclic ester as a mainsolvent and a cyclic ester are particularly suitable.

Apart from the esters exemplified above, the following solvents may alsobe used: acyclic phosphoric acid triesters such as trimethyl phosphate;ethers such as 1,2-dimethoxyethane, 1,3-dioxolane, tetrahydrofuran,2-methyl-tetrahydrofuran and diethyl ether; nitriles and dinitriles;isocyanates; and halogen-containing solvents. Furthermore, amine orimide organic solvents may be used.

Examples of the electrolyte salt to be dissolved in the solvent duringthe preparation of the nonaqueous electrolytic solution include LiC 10₄,LiBF₆, LiBF₄, LiAsF₆, LiSbF₆, LiCF₃SO₃, LiC₄F₉SO₃, LiCF₃CO₂,Li₂C₂F₄(SO₃)₂, LiN(RfSO₂)(Rf′SO₂), LiC(RfSO₂)₃, LiC_(a)F_(2n,+1)SO₃wherein n≧2, and LiN(RfOSO₂)₂ wherein Rf and Rf each represent afluoroalkyl group. They may be used alone or in combination of two ormore thereof Among these electrolyte salts, particularly preferred arefluorine-containing organic lithium salts having 2 or more carbon atoms,since such lithium salts have a large anionic property and further ionseparation easily occurs so that the salts are easily dissolved in theabove-mentioned solvents. The concentration of the electrolyte salt inthe nonaqueous electrolytic solution is not particularly limited, and itis preferably 0. 3 mol/L or more, more preferably 0.4 mol/L or more,while it is preferably 1.7 mol/L or less, more preferably 1.5 mol/L orless.

According to the present invention, the nonaqueous electrolyte shouldcontain a compound having at least two nitrile groups in the molecule.

The nitrile compound used according to the present invention can form aprotective film on the surface of the positive electrode active materialduring charging, in particular, initial charging, of the battery, andthe protective film suppresses the direct contact of the positiveelectrode and the nonaqueous electrolyte. Therefore, the battery of thepresent invention comprising the nonaqueous electrolyte containing thecompound having at least two nitrile groups in the molecule can preventthe generation of a gas in the battery due to the reaction of thepositive electrode and the nonaqueous electrolyte, when it is stored ata high temperature of, for example, about 85° C., in a charged state,since the protective film formed of the nitrile compound prevents thepositive electrode and the nonaqueous electrolyte from direct contactand in turn prevents the reaction therebetween. The gas generated by thereaction of the positive electrode and the nonaqueous electrolyte in thebattery causes the expansion of the battery resulting in the decrease ofthe battery characteristics due to the increase of the distance betweenthe positive and negative electrodes. However, the battery of thepresent invention can suppress the expansion of the battery caused bythe gas during storage and thus has good storage characteristics.

The compound having at least two nitrile groups in the molecule may beone having two nitrile groups in the molecule, one having three nitrilegroups in the molecule, etc. Among them, a dinitrile compound, that is,a compound having two nitrile groups in the molecule is preferable sinceit has a better property to form the protective film and thus to preventthe reaction of the positive electrode and the nonaqueous electrolyte.The dinitrile compound is preferably a compound of the formula: NC—R—CNwherein R is a linear or branched hydrocarbon group having 1 to 10carbon atoms, preferably a linear or branched alkylene group having 1 to10 carbon atoms.

Specific examples of the dinitrile compound include malononitrile,succinonitrile, glutaronitrile, adiponitrile, 1,4-dicyanoheptane,1,5-dicyanopentane, 1,6-dicyanohexane, 1,7-dicyanoheptane,2,6-dicyanoheptane, 1,8-dicyanooctane, 2,7-dicyanooctane, 1,9-dicyanononane, 2,8-dicyanononane, 1,10-dicyanodecane, 1,6-dicyanodecane,2,4-dimethylglutaronitrile, etc.

The nonaqueous electrolyte containing the nitrile compound may beprepared by any method. For example, the nitrile compound and theelectrolyte salt are dissolved in the solvent described above by aconventional method.

The amount of the nitrile compound is preferably at least 0.005% byweight, more preferably at least 0.01% by weight, still more preferablyat least 0.05% by weight, based on the whole weight of the nonaqueouselectrolyte, from the viewpoint of effectively utilizing the effects ofthe addition of the nitrile compound. However, when the amount of thenitrile compound in the electrolyte is too large, the charge-dischargecycle characteristics of the battery tends to decrease although thestorage characteristics of the battery is improve. Thus, the amount ofthe nitrile compound is preferably 1% or less, more preferably 0.75% byweight or less, still more preferably 0.5% by weight or less, based onthe whole weight of the nonaqueous electrolyte.

Besides the nitrile compound, the nonaqueous electrolytic solution maycontain other additive or additives. A preferred example of the additiveis a nonionic aromatic compound. Specific examples thereof includearomatic compounds having an alkyl group bonded to an aromatic ring(e.g., cyclohexylbenzene, isopropylbenzene, tert-butylbenzene,tert-amylbenzene, octylbenzene, toluene and xylene); aromatic compoundshaving a halogen group bonded to an aromatic ring (e.g., fluorobenzene,difluorobenzene, trifluorobenzene and chlorobenzene); aromatic compoundshaving an alkoxy group bonded to an aromatic ring (e.g., anisole,fluoroanisole, dimethoxybenzene and diethoxybenzene); aromaticcarboxylic acid esters such as phthalic acid esters (e.g., dibutylphthalate and di-2-ethylhexyl phthalate) and benzoic acid esters;carbonic acid esters having a phenyl group (e.g., methylphenylcarbonate, butylphenyl carbonate and diphenyl carbonate); phenylpropionate; and biphenyl. Among them, the compounds having an alkylgroup bonded to an aromatic ring (alkaryl compounds) are preferred, andcyclohexylbenzene is particularly preferred.

The aromatic compounds exemplified above can also form a film on thesurface of the active material in the positive electrode or the negativeelectrode in the battery. These aromatic compounds may be used alone,while more advantageous effects can be attained by the use of two ormore of the aromatic compounds together. Particularly advantageouseffects can be attained on the improvement of the safety of the batteryby the use of the alkaryl compound together with an aromatic compound,which can be oxidized at a lower voltage than the alkaryl compound, suchas biphenyl.

The method for adding the aromatic compound in the nonaqueouselectrolytic solution is not particularly limited. In general, thearomatic compound is added to the nonaqueous electrolytic solution priorto the fabrication of the battery. The content of the aromatic compoundin the nonaqueous electrolytic solution is preferably 4% by weight ormore from the viewpoint of the safety, and it is preferably 10% byweight or less from the viewpoint of the load characteristic. When twoor more aromatic compounds are used together, the total amount thereofis within the above-mentioned range. When the alkaryl (alkylaryl)compound and the aromatic compound which can be oxidized at a lowervoltage that the alkaryl compound are used in combination, the contentof the alkaryl compound in the nonaqueous electrolytic solution ispreferably 0.5% by weight or more, more preferably 2% by weight or more,while it is preferably 8% by weight or less, more preferably 5% byweight or less.

The content of the aromatic compound that can be oxidized at a lowervoltage than the alkaryl compound in the nonaqueous electrolyticsolution is preferably 0.1% by weight or more, more preferably 0.2% byweight or more, while it is preferably 1% by weight or less, morepreferably 0.5% by weight or less.

Furthermore, a surface protecting coating can be formed on the surfaceof the positive electrode active material in the step of initialcharging of the battery, when the nonaqueous electrolytic solutioncontains at least one compound selected from the group consisting ofhalogen-containing organic solvents (e.g., halogen-containingcarbonates), organic sulfur compounds, fluorine-containing organiclithium salts, phosphorus-containing organic solvents,silicon-containing organic solvents, nitrogen-containing organicsolvents other than the compounds having at least two nitrile groups inthe molecule, etc. In particular, the fluorine-containing organiccompounds such as fluorine-containing carbonate esters, the organicsulfur compounds and the fluorine-containing organic lithium salts arepreferred. Specific examples thereof include F-DPC[C₂F₅CH₂O(C═O)OCH₂C₂F₅], F-DEC [CF₃CH₂O(C═O)OCH₂CF₃], HFE7100(C₄F₉OCH₃), butyl sulfate (C₄H₉OSO₂OC₄H₉), methylethylene sulfate[(—OCH(CH₃)CH₂O—)SO₂], butyl sulfate (C₄H₉SO₂C₄H₉), a polymer imide salt([—N(Li)SO₂OCH₂(CF₂)₄CH₂OSO₂—]_(n) wherein n is from 2 to 100),(C₂F₅SO₂)₂NLi, and [(CF₃)₂CHOSO₂]₂NLi.

Such additives may be used alone. It is particularly preferable to use afluorine-containing organic solvent together with a fluorine-containingorganic lithium salt. The amount of such an additive added is preferably0.1% by weight or more, more preferably 2% by weight or more,particularly preferably 5% by weight or more, while it is preferably 30%by weight or less, more preferably 10% by weight or less, each based onthe whole weight of the nonaqueous electrolytic solution. When theamount of the additive is too large, the electric characteristics of thebattery may deteriorate. When the amount is too small, a good coatingmay hardly be formed.

When the battery comprising the nonaqueous electrolytic solutioncontaining the above-mentioned additive(s) is charged, particularly at ahigh voltage, a surface protecting coating that contains fluorine orsulfur atoms is formed on the positive electrode active materialsurface. This surface protecting coating may contain either fluorineatoms or sulfur atoms. Preferably, the coating contains both of fluorineatoms and sulfur atoms.

The amount of the sulfur atoms in the surface protecting coating formedon the positive electrode active material surface is preferably 0.5atomic % or more, more preferably 1 atomic % or more, particularlypreferably 3 atomic % or more. However, when the amount of the sulfuratoms in the positive electrode active material surface is too large,the discharge characteristic of the battery tends to decrease. Thus, theamount is preferably 20 atomic % or less, more preferably 10 atomic % orless, particularly preferably 6 atomic % or less.

The amount of the fluorine atoms in the surface protecting coatingformed on the positive electrode active material surface is preferably15 atomic % or more, more preferably 20 atomic % or more, particularlypreferably 25 atomic % or more. However, when the amount of the fluorineatoms in the positive electrode active material surface is too large,the discharge characteristic of the battery tends to fall. Thus, theamount of the fluorine atoms is preferably 50 atomic % or less, morepreferably 40 atomic % or less, particularly preferably 30 atomic % orless.

In order to improve the charge-discharge cycle characteristic of thebattery, preferably, the nonaqueous electrolytic solution contains atleast one carbonate compound selected from the group consisting ofvinylene carbonates, such as (—OCH═CHO—)C═O, (—OCH═C(CH₃)O—)C═O and(—OC(CH₃)═C(CH₃)O—)C═O, and derivatives thereof; cyclic carbonate estershaving a vinyl group such as vinylethylene carbonate(—OCH₂—CH(—CH═CH₂)O—) C═O); and fluorine-substituted ethylenecarbonates, such as (—OCH₂—CHFO—)C═O and (—OCHF—CHFO—)C═O. The additionamount thereof is preferably 0.1% by weight or more, more preferably0.5% by weight or more, particularly preferably 2% by weight or morebased on the whole weight of the nonaqueous electrolytic solution. Whenthe addition amount thereof is too large, the load characteristic of thebattery tends to decrease. Thus, the addition amount is preferably 10%by weight or less, more preferably 5% by weight or less, particularlypreferably 3% by weight or less based on the whole weight of thenonaqueous electrolytic solution.

In the present invention, the nonaqueous electrolyte may be a gel-formpolymer electrolyte besides the nonaqueous electrolytic solutiondescribed above. The gel-form polymer electrolyte corresponds to aproduct obtained by the gelation of the nonaqueous electrolytic solutionwith a gelling agent. For the gelation of the nonaqueous electrolyticsolution, the following gelling agents may be used: a linear polymersuch as polyvinylidene fluoride, polyethylene oxide orpolyacrylonitrile, or a copolymer thereof; or a polyfunctional monomerwhich can be polymerized by irradiation with actinic rays such asultraviolet rays or electron beams (e.g., an acrylate having 4 or morefunctionalities such as pentaerythritol tetraacrylate,ditrimethylolpropane tetraacrylate, ethoxylated pentaerythritoltetraacrylate, dipentaerythritol hydroxypentaacrylate, ordipentaerythritol hexaacrylate; or a methacrylate having 4 or morefunctionalities, which are analogous to the above acrylates. In the caseof the monomer, the monomer itself does not cause the gelling of theelectrolytic solution, but the polymer formed from the monomer acts as agelling agent.

When a polyfunctional monomer is used to gel the electrolytic solutionas described above, a polymerization initiator may optionally be used.Examples of the polymerization initiator include benzoyls, benzoin alkylethers, benzophenones, benzoylphenylphosphine oxides, acetophenones,thioxanthones and anthraquinones. As a sensitizer for the polymerizationinitiator, an alkylamine or an aminoester may be used.

In the present invention, the nonaqueous electrolyte may be a solidelectrolyte besides the nonaqueous electrolytic solution or the gel-formpolymer electrolyte. The solid electrolyte may be an inorganic solidelectrolyte or an organic solid electrolyte.

The positive electrode according to the present invention may have astructure such that the positive electrode mixture layer containing thepositive electrode active material is formed on one or both of thesurfaces of an electrode collector.

The positive electrode mixture layer according to the present inventionmay have a density of 3.5 g/cm³ or more, more preferably 3.6 g/ cm³ ormore, particularly preferably 3.8 g/ cm³ or more. With such a density,the capacity of the battery can be increased. However, when the densityof the positive electrode mixture layer is too high, the wettabilitywith the nonaqueous electrolyte, which will be explained later,decreases. Thus, the density is preferably 4.6 g/ cm³ or less, morepreferably 4.4 g/ cm³ or less, particularly preferably 4.2 g/ cm³ orless.

Herein, the density of the positive electrode mixture layer may beobtained by the following measuring method: The positive electrode iscut to form a sample piece having a predetermined area, the sample pieceis weighed with an electronic balance having a minimum scale of 1 mg,and then the weight of the current collector is subtracted from theweight of the sample piece to calculate the weight of the positiveelectrode mixture layer. The total thickness of the positive electrodeis measured at ten points with a micrometer having a minimum scale of 1μm. Then, the thickness of the current collector is subtracted from theresultant individual thicknesses, and the thicknesses of the positiveelectrode mixture layer measured at ten points are averaged. From theaveraged thicknesses of the positive electrode mixture layer and thesurface area, the volume of the positive electrode mixture layer iscalculated. Finally, the weight of the positive electrode mixture layeris divided by the volume thereof to obtain the density of the positiveelectrode mixture layer.

The positive electrode active material contained in the positiveelectrode mixture layer comprises a lithium-containing transition metaloxide comprising at least one metal element selected from the groupconsisting of Mg, Ti, Zr, Ge, Nb, Al and Sn. The use of thelithium-containing transition metal oxide can improve thecharge-discharge cycle characteristics of the battery, since it has goodstability, in particular, in the charged state at a high voltage, sothat the decay of the compound is prevented when the charge-dischargecycle is repeated. Since the lithium-containing transition metal oxidehas good stability, it can improve the storage characteristics and thereliability such as safety of the battery.

The positive electrode active material preferably has a particle sizefrequency peak in a particle size range larger than a midpoint d_(m)between d₁₀ and d₉₀ in a particle size distribution curve, whichobtained by a method for measuring an average particle size describedbelow. To obtain a positive electrode active material having such aparticle size distribution, preferably, at least two lithium-containingtransition metal oxides having different average particle sizes areused. When a lithium-containing transition metal oxide having a largeraverage particle size and one having a smaller average particle size areused in combination, the particles of the latter infill the gaps amongthe particles of the former. Thus, the positive electrode mixture layerhaving a large density as described above can be easily formed.

The “average particle size” of the lithium-containing transition metaloxide(s) used herein means a 50% diameter value (d₅₀) , that is, anmedian diameter, read from an integral fraction curve based on volumes,which is obtained by integrating the volumes of the particles from asmaller particle size measured by a MICROTRAC particle size analyzer(HRA 9320 available from NIKKISO Co., Ltd.). Analogously, d₁₀ and d₉₀means a 10% diameter value and a 90% diameter value, respectively.

The mixture of “at least two lithium-containing transition metal oxideshaving different average particle sizes” preferably has a particle sizefrequency peak in a particle size range larger than a midpoint d_(M)between d₁₀ and d90 in a particle size distribution curve, as describedabove. Hereinafter, a particle size corresponding to the particle sizefrequency peak is expressed by “d_(p)” . In a preferred embodiment, theratio of d_(p) to d_(M) (d_(p)/d_(M)) is at least 1.05, more preferablyat least 1.2, most preferably at least 1.3. Furthermore, the ratio ofd_(p) to d_(M) is preferably 1.6 or less, more preferably 1.5 or less,most preferably 1.45 or less. In a more preferred embodiment, thelithium-containing transition metal oxide mixture has at least two peaksin the particle size distribution curve. When the lithium-containingtransition metal oxides have the same d_(p)/d_(M) of 1.3, thelithium-containing transition metal oxide mixture having at least twopeaks in the particle size distribution curve can increase the densityof the positive electrode mixture layer by 0.1 g/cm³ or more. When theparticle size distribution curve has two or more peaks, it can bedivided into a distribution curve corresponding to particles having alarger particle size and a distribution curve corresponding particleshaving a smaller particle size by a common peak separation method andthen an average particle size (d₅₀) of the respective lithium-containingtransition metal oxide and a mixing ratio of the lithium-containingtransition metal oxides having the different average particle sizes canbe calculated from the particle sizes and the cumulative volumes of theparticles.

When the average particle size of the lithium-containing transitionmetal oxide having the largest average particle size (hereinafterreferred to as “positive electrode active material (A)”) is representedby A, and that of the lithium-containing transition metal oxide havingthe smallest average particle size (hereinafter referred to as “positiveelectrode active material (B)”) is expressed by B, the ratio of B to A(i.e., B/A) is preferably from 0.15 to 0.6. When the average particlesizes of the two positive electrode active materials (A) and (B) havesuch a ratio B/A, the density of the positive electrode mixture layercan be easily increased.

The positive electrode active material (A) preferably has an averageparticle size of 5 μm or more, more preferably 8 μm or more,particularly preferably 11 μm or more. When the average particle size ofthe positive electrode active material (A) is too small, the density ofthe positive electrode mixture layer may hardly be increased. When theaverage particle size is too large, the battery characteristic tends todecrease. Thus, the average particle size is preferably 25 μm or less,more preferably 20 μm or less, particularly preferably 18 μm or less.

The positive electrode active material (B) preferably has an averageparticle size of 10 μm or less, more preferably 7 μm or less,particularly preferably 5 μm or less. When the average particle size ofthe positive electrode active material (B) is too large, the positiveelectrode active material (B) does not easily fill the gaps between theparticles of the lithium-containing transition metal oxide having arelatively large particle size in the positive electrode mixture layer,so that the density of this layer may hardly be increased. When theaverage particle size is too small, the volume of voids among the smallparticles increases so that the density of the positive electrodemixture layer may not be increased. Thus, the average particle size ofthe positive electrode active material (B) is preferably 2 μm or more,more preferably 3 μm or more, particularly preferably 4 μm or more.

The positive electrode active materials according to the presentinvention may contain only two lithium-containing transition metaloxides having different average particle sizes, for example, thepositive electrode active materials (A) and (B) as descried above, whilethe positive electrode active materials may contain three or more, forexample, three, four or five lithium-containing transition metal oxideshaving different average particle sizes, for example, the positiveelectrode active materials (A) and (B) and one or morelithium-containing transition metal oxides having an average particlesize between those of the positive electrode active materials (A) and(B).

The content of the positive electrode active material (B) having thesmallest average particle size in the lithium-containing transitionmetal oxides contained in the positive electrode is preferably 5% byweight or more, more preferably 10% by weight or more, particularlypreferably 20% by weight or more. When the positive electrode activematerial (B) is contained in an amount of the above-mentioned range, thegaps between the particles of the lithium-containing transition metaloxide having a relatively large particle size are easily filledtherewith so that the density of the positive electrode mixture layer isincreased. When the content of the positive electrode active material(B) is too large, the density of the positive electrode mixture layer ishardly be increased. Thus, the content of the positive electrode activematerial (B) is preferably 60% by weight or less, more preferably 50% byweight or less, particularly preferably 40% by weight or less.

Accordingly, when the lithium-containing transition metal oxidescontained in the positive electrode are only the positive electrodeactive materials (A) and (B), the content of the positive electrodeactive material (A) is preferably 40% by weight or more, more preferably50% by weight or more, particularly preferably 60% by weight or more ofthe oxides, while it is preferably 95% by weight or less, morepreferably 90% by weight or less, particularly preferably 80% by weightor less.

Among the lithium-containing transition metal oxides contained in thepositive electrode, the positive electrode active material (B) havingthe smallest average particle size has the above-mentioned averageparticle size. Such a lithium-containing transition metal oxide having arelatively small particle size has low stability, for example, in astate that the battery is charged at a high voltage, so that the oxidemay damage the reliability including the safety of the battery.

When two or more lithium-containing transition metal oxides having thedifferent average particle sizes are used, a lithium-containingtransition metal oxide comprising at least one metal element M² selectedfrom the group consisting of Mg, Ti, Zr, Ge, Nb, Al and Sn is preferablyused at least as the positive electrode active material (B) which is thelithium-containing transition metal oxide having the smallest averageparticle size, since the lithium-containing transition metal oxidecomprising the metal element M² has the improved stability and thussurely increases the charge-discharge cycle characteristics of thebattery and also improves the storage characteristics and thereliability including the safety of the battery.

When two or more lithium-containing transition metal oxides having thedifferent average particle sizes are used, the positive electrode activematerial (B) having the smallest average particle size preferablycomprises the metal element M². More preferably, the lithium-containingtransition metal oxide other than the positive electrode active material(B), for example, the positive electrode active material (A) having thelargest average particle size or the lithium-containing transition metaloxide having the average particle size between those of the positiveelectrode active materials (A) and (B), comprises the metal element W.When the lithium-containing transition metal oxide other than thepositive electrode active material (B) comprises the metal element M²,the reliability including the safety of the battery can be furtherimproved, since such a lithium-containing transition metal oxide has theimproved stability, in particular, the stability in a state that thebattery is charged at a high voltage as described above.

The positive electrode active material (B) is preferably alithium-containing transition metal oxide represented by the followingformula (1):

Li_(x)M ¹ _(y)M² _(z)M³ _(v)O₂   (1)

wherein M¹ represents at least one transition metal element selectedfrom Co, Ni and Mn, M² represents at least one metal element selectedfrom the group consisting of Mg, Ti, Zr, Ge, Nb, Al and Sn, M³represents an element other than Li, M¹ and M², and x, y, z and v arenumbers satisfying the following equations respectively: 0.97≦x<1.02,0.8≦y<1.02, 0.002≦z≦0.05, and 0≦v≦0.05. z is preferably at least 0.004,more preferably at least O. 006, while it is preferably less than 0.02,more preferably less than 0.01. When z is too small, thecharge-discharge characteristics or the safety of the battery may notsufficiently be improved. When z is too large, the electriccharacteristics of the battery tend to deteriorate.

Each of the lithium-containing transition metal oxides other than thepositive electrode active material (B) such as the positive electrodeactive material (A) is preferably a lithium-containing transition metaloxide represented by the following formula (2):

Li_(a)M¹ _(b)M² _(c)M³ _(d)O₂   (2)

wherein M¹, M² and M³ are the same as defined in the formula (1), and a,b, c and d are numbers satisfying the following equations respectively:0.97 <a <1.02, 0.8 <b <1.02, 0 <c <0.02, and 0 <d <0.02.

M¹, M² and M³ are selected from the same elements as in the formula (1),but the elements selected or the constituting element ratios selected inthe individual positive electrode active materials having differentaverage particle sizes may differ from each other. For example, in thepositive electrode active material (B), Mg, Ti and Al may be selected,while in the positive electrode active material (A), Mg and Ti may beselected. As explained in this example, however, among the elements M²,preferably at least one common element is selected, more preferably atleast two common elements are selected, and particularly preferably atleast three common elements are selected.

In the case of the positive electrode active material (A), “c” ispreferably 0.0002 or more, more preferably 0.001 or more, and it ispreferably less than 0.005, more preferably less than 0.0025, and “d” ispreferably 0.0002 or more, more preferably 0.001 or more and it ispreferably less than 0.005, more preferably less than 0.0025 for thefollowing reason: the particle size of the positive electrode activematerial (A) is relatively large; thus, when the amount of M² and thelike added to the material (A) is relatively small, advantageous effectscan be attained; but when the amount is too large, the electricalcharacteristics of the battery tends to decrease.

The transition metal element in the lithium-containing transition metaloxide is preferably mainly Co and/or Ni. For example, the total amountof Co and Ni is preferably 50% by mole or more based on all thetransition metal elements contained in the lithium-containing transitionmetal oxides.

Preferably, the proportion of Co in the lithium-containing transitionmetal oxide is higher, since the density of the positive electrodemixture layer can be made higher. In the formulae (1) and (2), theproportion of Co in the transition metal element M¹ is preferably 30% bymole or more, more preferably 65% by mole or more, particularlypreferably 95% by mole or more.

The values of x in the formula (1) and a in the formula (2) may vary asthe battery is charged or discharged. Nevertheless, when the battery isan as-produced one, x and a are each preferably 0.97, more preferably0.98 or more, particularly preferably 0.99 or more, while x and a areeach preferably less than 1.02, more preferably 1.01 or less,particularly preferably 1.00 or less.

The values of y in the formula (1) and b in the formula (2) are eachpreferably 0.98 or more, more preferably 0.98 or more, particularlypreferably 0.99 or more, and they are each preferably less than 1.02,more preferably less than 1.01, particularly preferably less than 1.0.

Each of the positive electrode active material (B) represented by theformula (1), and the lithium-containing transition metal oxides otherthan the positive electrode active material (B) which are represented bythe formula (2) preferably contains Mg as the element M², since thesafety of the battery is more effectively improved. In addition, each ofthem comprises Mg and also at least one metal element M² selected fromthe group consisting of Ti, Zr, Ge, Nb, Al and Sn. In this case, thestability of those lithium-containing transition metal oxides is furtherimproved in a state that the battery is charged at a high voltage.

In the positive electrode active material (B), the content of Mg ispreferably at least 0.1% by mole, more preferably at least 0.15% bymole, particularly preferably at least 0.2% by mole, based on the amountof the metal element MI, from the view point of more effectivelyattaining the effects of Mg.

When the positive electrode active material (B) contains at least onemetal element selected from Ti, Zr, Ge and Nb, the total content thereofis at least 0.05% by mole, more preferably at least 0.08% by mole,particularly preferably at least 0.1% by mole, based on the content ofM¹, from the viewpoint of more effectively attaining the effects of theuse of these metal elements. When the positive electrode active material(B) contains Al and/or Sn, the total content thereof is preferably 0.1%by mole or more, more preferably 0.15% by mole or more, particularlypreferably 0.2% by mole or more based on the content of M¹, from theviewpoint of more effectively attaining the effects of the use of thesemetal elements.

When the content of Mg is too large in the positive electrode activematerial (B), the load characteristic of the battery tends to decrease.Therefore, the content of Mg is preferably less than 2% by mole, morepreferably less than 1% by mole, particularly preferably 0.5% by mole,most preferably 0.3% by mole.

When the content of Ti, Zr, Ge, Nb, Al and/or Sn is too large in thepositive electrode active material (B) , the effect to increase thecapacity of the battery may not be sufficient. Thus, when the oxidecontains Ti, Zr, Ge and/or Nb, the total content thereof is preferablyless than 0.5% by mole, more preferably less than 0.25% by mole,particularly preferably less than 0.15% by mole based on the content ofM¹. When the oxide contains Al and/or Sn, the total amount thereof ispreferably less than 1% by mole, more preferably less than 0.5% by mole,particularly preferably less than 0.3% by mole based on the content ofM¹.

In the positive electrode active material (A), the content of Mg ispreferably at least 0.01% by mole, more preferably at least 0.05% bymole, particularly preferably at least 0.07% by mole, based on theamount of the metal element M¹, from the view point of more effectivelyattaining the effects of Mg.

When the positive electrode active material (A) contains at least onemetal element selected from Ti, Zr, Ge and Nb, the total content thereofis at least 0.005% by mole, more preferably at least 0.008% by mole,particularly preferably at least 0.01% by mole, based on the content ofMI, from the viewpoint of more effectively attaining the effects of theuse of these metal elements. When the positive electrode active material(A) contains Al and/or Sn, the total content thereof is preferably 0.01%by mole or more, more preferably 0.05% by mole or more, particularlypreferably 0.07% by mole or more based on the content of M¹, from theviewpoint of more effectively attaining the effects of the use of thesemetal elements.

Also, when the content of Mg is too large in the positive electrodeactive material (A), the load characteristic of the battery tends todecrease. Therefore, the content of Mg is preferably less than 0.5% bymole, more preferably less than 0.2% by mole, particularly preferably0.1% by mole.

Also, when the content of Ti, Zr, Ge, Nb, Al and/or Sn is too large inthe positive electrode active material (A), the effect to increase thecapacity of the battery may not be sufficient. Thus, when the oxidecontains Ti, Zr, Ge and/or Nb, the total content thereof is preferablyless than 0.3% by mole, more preferably less than 0.1% by mole,particularly preferably less than 0.05% by mole based on the content ofM¹. When the oxide contains Al and/or Sn, the total amount thereof ispreferably less than 0.5% by mole, more preferably less than 0.2% bymole, particularly preferably less than 0.1% by mole based on thecontent of M¹.

Furthermore, when the lithium-containing transition metal oxide otherthan the positive electrode active materials (A) and (B) is used, thecontent of Mg in the other metal oxide is preferably at least 0.01% bymole, more preferably at least 0.05% by mole, particularly preferably atleast 0.07% by mole, based on the amount of the metal element M¹, fromthe view point of more effectively attaining the effects of Mg.

When the lithium-containing transition metal oxide other than thepositive electrode active materials (A) and (B) contains at least onemetal element selected from Ti, Zr, Ge and Nb, the total content thereofis at least 0.005% by mole, more preferably at least 0.008% by mole,particularly preferably at least 0.01% by mole, based on the content ofM¹, from the viewpoint of more effectively attaining the effects of theuse of these metal elements. When the positive electrode active material(A) contains Al and/or Sn, the total content thereof is preferably 0.01%by mole or more, more preferably 0.05% by mole or more, particularlypreferably 0.07% by mole or more based on the content of M¹, from theviewpoint of more effectively attaining the effects of the use of thesemetal elements.

However, again in the lithium-containing transition metal oxide otherthan the positive electrode active materials (A) and (B), when thecontent of Mg is too large, the load characteristic of the battery tendsto decrease. Therefore, the content of Mg is preferably less than 2% bymole, more preferably less than 1% by mole, particularly preferably 0.5%by mole, most preferably 0.3% by mole, each based on the content of M¹.

Again, in the lithium-containing transition metal oxide other than thepositive electrode active materials (A) and (B), when the content of Ti,Zr, Ge, Nb, Al and/or Sn is too large in the positive electrode activematerial (A), the effect to increase the capacity of the battery may notbe sufficient. Thus, when the oxide contains Ti, Zr, Ge and/or Nb, thetotal content thereof is preferably less than 0.5% by mole, morepreferably less than 0.25% by mole, particularly preferably less than0.15% by mole based on the content of M¹. When the oxide contains Aland/or Sn, the total amount thereof is preferably less than 1% by mole,more preferably less than 0.5% by mole, particularly preferably lessthan 0.3% by mole based on the content of M.

A method for including the metal element M² in the positive electrodeactive material (B) or the other lithium-containing transition metaloxide(s) is not particularly limited. For example, the element M² may bepresent on the particles of the metal oxide, may be evenly present as asolid solution inside the metal oxides, or may be unevenly presentinside the metal oxides with having a density distribution. Furthermore,the element M² may form a compound which in turn forms a layer on theparticle surfaces. Preferably, the element M² is evenly present as asolid solution.

In the formulae (1) and (2) representing the positive electrode activematerial (B) and the other lithium-containing transition metal oxide(s),respectively, the element M³ is an element other than Li, M¹ and M². Thepositive electrode active material (B) and the other lithium-containingtransition metal oxides may each contain the M³ in an amount such thatthe advantageous effects of the present invention are not impaired, orthey may contain no M³.

Examples of the element M³ include alkali metals other than Li (e.g.,Na, K and Rb) , alkaline earth metals other than Mg (e.g., Be, Ca, Srand Ba) , Group Ilia metals (e.g., Sc, Y, La) , Group IVa metals otherthan Ti and Zr (e.g., Hf) , Group Va metals other than Nb (e.g., V andTa) , Group VIa metals (e.g., Cr, Mo and W), Group VIIb metals otherthan Mn (e.g., Tc and Re) , Group VIII metals other than Co and Ni(e.g., Fe, Ru, and Rh), Group lb metals (e.g., Cu, Ag and Au), GroupIIIb metals other than Zn and Al (e.g ., B, Ca and In), Group IVb metalsother than Sn and Pb (e.g., Si), P and Bi.

The metal element M² contributes to an improvement in the stability ofthe lithium-containing transition metal oxides. However, when thecontent thereof is too large, a function of storing and releasing Liions is impaired so that the battery characteristics may be decreased.Since the positive electrode active material (B) having the smallestaverage particle size has the particularly small particle size anddecreased stability, it is preferable that the content of the elementM², which is a stabilizing element, is somewhat high. In addition, sincethe positive electrode active material (B) has the small particle sizeand in turn the large surface area, it exhibits a high activity. Thus,the presence of the element M² in the material (B) has less influence onthe function of storing and releasing Li ions.

In contrast, the lithium-containing transition metal oxides havingrelatively large particle sizes, that is, the lithium-containingtransition metal oxides other than the positive electrode activematerial (B), have better stability than the positive electrode activematerial (B). Therefore, the former metal oxides have less necessity tocontain the element M² than the positive electrode active material (B).Furthermore, their function of storing and releasing Li ions is easilyimpaired by the presence of the element M² since the materials have thesmaller surface area and the lower activity than the positive electrodeactive material (B).

Accordingly, it is preferable that the content of the metal element M²in the positive electrode active material (B) is larger than that in thelithium-containing transition metal oxide(s) other than the positiveelectrode active material (B).

That is, z in the formula (1) is preferably larger than c in the formula(2) (z>c) . In particular, z is at least 1.5 times, more preferably atleast 2 times, particularly preferably at least 3 times larger than c.When z is much larger than c, the load characteristics of the batterytend to decrease. Thus, z is preferably less than 5 times as large asc¹, more preferably less than 4 times as large as c, particularlypreferably less than 3.5 times as large as c.

When the three or more lithium-containing transition metal oxides havingthe different average particle sizes are contained in the positiveelectrode, there is no especial limitation on the relationship of theelement M² content between the positive electrode active material (A)having the largest average particle size and the otherlithium-containing transition metal oxides. Thus, the former may containa larger amount of the element M² than the latter, and vise versa, orthe element M² contents in the former and the latter may be the same. Ina more preferable embodiment, a metal oxide having a smaller averageparticle size contains a larger amount of the element M². In particular,when the three lithium-containing transition metal oxides havingdifferent average particle sizes are used, the element M² content in thepositive electrode active material (B) having the smallest averageparticle size is largest, that in lithium-containing transition metaloxide having the average particle size between those of the activematerials (A) and (B) is second largest, and that in the positiveelectrode active material (A) having the largest average particle sizeis smallest.

When two or more lithium-containing transition metal oxides are used,the oxides having different average particle sizes may have the samecomposition of elements, or different compositions of elements betweenthem. When the lithium-containing transition metal oxides according tothe present invention are the above-mentioned positive electrode activematerials (A) and (B), the following combination may be used: acombination of the positive electrode active material (A) consisting ofLiCoo.998Mgo.o008Tio.0oo4Alo.oo0s02, and the positive electrode activematerial (B) consisting ofLiCo_(0.334)Ni_(0.33)Mn_(0.33)Mg_(0.0024)Ti₀.₀₀₁₂Al_(0.0024)O₂.

The positive electrode active material, namely, the lithium-containingtransition metal oxide used according to the present invention is formedthrough a certain synthesizing process and a certain battery producingprocess. For example, for the preparation of lithium-containingtransition metal oxides which contain Co as the transition metal elementM¹ and have different average particle sizes, firstly, a solution of analkali such as NaOH is dropwise added to an acidic aqueous solutioncontaining Co to precipitate Co(OH)₂. In order to homogeneouslyprecipitate Co(OH)₂, Co may be coprecipitated with a different element,and then the coprecipitated material is calcined to obtain Co₃0₄. Theparticle size of the precipitates can be adjusted by controlling theperiod for forming the precipitates. The particle size of Co₃0₄ aftercalcination is also controlled by the particle size of the precipitateat this time.

When the positive electrode active material is synthesized, conditionssuch as a mixing condition, calcination temperature, calcinationatmosphere, calcination time, starting materials, and also batteryfabrication conditions are suitably selected. With regard to the mixingcondition in the synthesis of the positive electrode active material,preferably, for example, ethanol or water is added to the powderystarting materials, and then mixed in a planetary ball mill for 0.5 houror longer. More preferably, ethanol and water are mixed at a volumeratio of 50:50, and the mixture is agitated in a planetary ball mill for20 hours or longer. Through this mixing step, the powdery startingmaterials are sufficiently comminuted and mixed to prepare a homogeneousdispersion. The dispersion is dried with a spray drier or the like whilekeeping homogeneity. The calcination temperature is preferably from 750to 1,050° C., more preferably from 950 to 1,030° C. The calcinationatmosphere is preferably an air. The calcination time is preferably from10 to 60 hours, more preferably from 20 to 40 hours.

In the preparation of the positive electrode active material, Li₂CO₃ ispreferably used as a lithium source. As the sources of other metal suchas Mg, Ti, Ge, Zr, Nb, Al and Sn, preferred are nitrates or hydroxidesof these metals, or oxides thereof having a particle size of 1 μm orless. It is preferable to use the coprecipitate of the hydroxides sincethe different elements are uniformly distributed in the active material.

The contents of the metal elements in the positive electrode activematerials are measured by the ICP atomic emission spectroscopy or thelike. The content of lithium can be measured by an atomic absorptionanalysis. In the state and one having a smaller particle size.Therefore, the contents or content ratios of a positive electrode, it isdifficult to separately measure the contents of the metal elements ineach of the positive electrode active material having a larger particlesize of the metal elements of the positive electrode active materialhaving different particle sizes may be measured with an electron provemicroanalyzer using a mixture of positive electrode active materialshaving a known mixing ratio as a standard sample. Alternatively, thepositive electrode is treated with a suitable solvent such asN-methyl-2-pyrrolidone (NMP) to separate the active material particlesfrom the positive electrode and settled out in the solvent, followed bywashing and drying. Then, the particle size distribution of therecovered particles is measured and the peak-separation of the particlesize distribution curve is carried out. When the inclusion of two ormore particles having different particle sized is confirmed, theparticles are classified into a larger one and a smaller one, and thecontents of the metal elements in each particle group are measured bythe ICP atomic emission spectroscopy.

Herein, the contents of the metal elements in the positive electrodeactive material may be measured by the ICP atomic emission spectroscopyas follows: about 5 g of the active material is precisely weighed andcharged in a 200 ml beaker. Then, 100 ml of aqua regia is added, and themixture is concentrated by heating to a liquid volume of about 20 to 25ml. After cooling, the mixture is filtrated through a quantitativefilter paper (No. 5B available from Advantec MFS, Inc.) to separate thesolids. The filtrate and washing liquid are charged in a 100 mlmeasuring flask and diluted to a specific volume. Then, the contents ofthe metal elements in the solution are measured with a sequential typeICP analyzer (IPIS 1000 manufactured by Nippon Jarrel-Ash Co., Ltd.).

When the content (I) of at least one metal element selected from thegroup consisting of Mg, Ti, Zr, Ge, Nb, Al and Sn in the transitionmetal oxide having the smallest average particle size and a content (II)of the same metal element as one with which the content (I) in thelithium-containing transition metal oxide other than one having thesmallest average particle size are measured by the ICP atomic emissionspectroscopy described above, the ratio of the content (I) to thecontent (II) corresponds to the relationship between z in the formula(I) and c in the formula (2). The ratio of the content (I) to thecontent (II) is preferably at least 1.5, more preferably at least 2,particularly preferably at least 3. Since the load characteristics ofthe battery tend to decrease when z is much larger than c, the ratio ofthe content (I) to the content (II) is preferably less than 5, morepreferably less than 4, particularly preferably less than 3.5.

The positive electrode used in the present invention is formed by, forexample, a method described below. Firstly, if necessary, an electricconductive aid (e.g., graphite, carbon black, acetylene black, etc.) isadded to the lithium-containing transition metal oxide used as apositive electrode active material. Furthermore, to the mixture, abinder (e.g., polyvinylidene fluoride, poly tetrafluoroethylene, etc.)is added to prepare a positive electrode mixture. A solvent is used toformulate this positive electrode mixture in the form of a paste. Thebinder may be mixed with the positive electrode active material and thelike after the binder is dissolved in a solvent. In this way, the pastecontaining the positive electrode mixture is prepared. The resultantpaste is applied to a positive electrode current collector made of analuminum foil or the like, and then dried to form a positive electrodemixture layer. If necessary, the layer is pressed to obtain a positiveelectrode. When the positive electrode active material comprises two oremore lithium-containing transition metal oxides having the differentaverage particle sizes, for example, the positive electrode activematerials (A) and (B), they are mixed at a predetermined ratio, and thenthe electric conductive aid and the binder are added to the mixture toprepare a positive electrode mixture, which is used in the subsequentsteps. However, the method for producing the positive electrode is notlimited to the above-mentioned method, and may be any other method.

The thickness of the positive electrode mixture layer is preferably from30 to 200 μm, and the thickness of the current collector used in thepositive electrode is preferably from 8 to 20 μm.

In the positive electrode mixture layer, the content of thelithium-containing transition metal oxides as the active materials ispreferably 96% by weight or more, more preferably 97oby weight or more,particularly preferably 97 .5% by weight or more, while it is preferably99% by weight or less, more preferably 98% by weight or less. Thecontent of the binder in the positive electrode mixture layer ispreferably 1% by weight or more, more preferably 1.3% by weight or more,particularly preferably 1.5% by weight or more, while it is preferably4% by weight or less, more preferably 3% by weight or less, particularlypreferably 2% by weight or less. The content of the electric conductiveaid in the positive electrode mixture layer is preferably 1% by weightor more, more preferably 1.1% by weight or more, particularly preferably1 .2% by weight or more, while it is preferably 3% by weight or less,more preferably 2% by weight or less, particularly preferably 1.5% byweight or less.

When the content of the active material in the positive electrodemixture layer is too small, the capacity cannot be increased and alsothe density of the positive electrode mixture layer cannot be increased.When this content is too large, the resistance may increase or theformability of the positive electrode may be impaired. When the bindercontent in the positive electrode mixture layer is too large, thecapacity may hardly be increased. When this content is too small, theadhesion of the layer to the current collector decreases so that thepowder may drop off from the electrode. Thus, the above-mentionedpreferable ranges are desirable. Furthermore, when the content of theelectric conductive aid in the positive electrode mixture layer is toolarge, the density of the positive electrode mixture layer may not bemade sufficiently high so that the capacity may hardly be increased.When this content is too small, the sufficient electric conductionthrough the positive electrode mixture layer is not attained so that thecharge-discharge cycle characteristic or the load characteristic of thebattery may deteriorated.

It is essential for the nonaqueous secondary battery of the presentinvention to have the nonaqueous electrolyte and the positive electrode,which are explained above, and thus there is no specific limitation onother elements or structure of the battery. The battery of the presentinvention may adopt various elements and structures, which are commonlyadopted in the conventional nonaqueous secondary batteries in the stateof art.

The negative electrode active material in the negative electrode may beany material that can be doped and de-doped with Li ions. Examplesthereof are carbonaceous materials such as graphite, pyrolytic carbons,cokes, glassy carbons, burned bodies of organic polymers, mesocarbonmicrobeads, carbon fibers and activated carbon. In addition, thefollowing materials can also be used as the negative electrode activematerial: alloys of Si, Sn, In or the like, oxides of Si, Sn or the likethat can be charged and discharged at a low voltage near a voltage atwhich Li can be charged and discharged, and nitrides of Li and Co suchas Li₂.₆Co₀.₄N. Graphite can be partially substituted with a metal, ametal oxide or the like that can be alloyed with Li. When graphite isused as the negative electrode active material, the voltage when thebattery is fully charged can be regarded as about 0.1 V with referenceto the potential of lithium, and therefore the voltage of the positiveelectrode can be conveniently calculated as a voltage obtained by adding0.1 V to the battery voltage. Consequently, the charge voltage of thepositive electrode is easily controlled.

Preferably, graphite has such a form in that a lattice spacing d₀₀₂ ofthe (002) planes is 0.338 nm or less, since the negative electrode or anegative electrode mixture layer, which will be explained later, has ahigher density as the crystallinity is higher. However, when the latticespacing d₀₀₂ is too large, the high density negative electrode maydecrease the discharge characteristic or the load characteristic of thebattery. Thus, the lattice spacing d₀₀₂ is preferably 0.335 nm or more,more preferably 0.3355 nm or more.

The crystal size of the graphite in the c axis direction (Lc) ispreferably 70 nm or more, more preferably 80 nm or more, particularlypreferably 90 nm or more. As the Lc is larger, the charging curvebecomes flat so that the voltage of the positive electrode is easilycontrolled and also the capacity can be made large. When the Lc is toolarge, the battery capacity tends to decrease with the high-densitynegative electrode. Thus, the Lc is preferably less than 200 nm.

Furthermore, the specific surface area of the graphite is preferably 0.5m²/g or more, more preferably 1 m²/g or more, particularly preferably 2m²/g or more, while it is preferably 6 m²/g or less, more preferably 5m²/g or less. Unless the specific surface area of the graphite issomewhat large, the characteristics tend to decrease. When the specificsurface area is too large, the graphite easily reacts with theelectrolyte and such a reaction may have influences on the properties ofthe battery.

The graphite used in the negative electrode is preferably made ofnatural graphite. More preferred is a mixture of two or more graphitematerials having different surface crystallinity to achieve the highdensity of the negative electrode. Since natural graphite is inexpensiveand achieves a high capacity, the negative electrode with a high costperformance can be produced. Usually, when natural graphite is used, thebattery capacity is easily decreased as the density of the negativeelectrode is increased. However, the decrease in the battery capacitycan be suppressed by mixing the natural graphite with a graphite havinga reduced surface crystallinity by a surface treatment.

The surface crystallinity of specific graphite can be determined by theRaman spectrum analysis. When the R value of the Raman spectrum(R=I₁₃₅₀I₁₅₈₀, that is, the ratio of the Raman intensity around 1350cm⁻¹ to that around 1580 cm⁻¹) is 0.01 or more, where the Raman spectrumis measured with graphite which has been excited with an argon laserhaving a wavelength of 514.5 nm, the surface crystallinity of thespecific graphite is slightly lower than that of natural graphite. Thus,with the graphite having a surface crystallinity decreased by thesurface treatment, the R value is preferably 0.01 or more, morepreferably 0.1 or more, while it is preferably 0.5 or less, morepreferably 0.3 or less. The content of the graphite having a surfacecrystallinity decreased by surface treatment is preferably 100% byweight of the whole graphite in order to increase the density of thenegative electrode. However, in order to prevent the decrease of thebattery capacity, the content of such graphite is preferably 50% byweight or more, more preferably 70% by weight or more, particularlypreferably 85% by weight or more of the whole graphite.

When the average particle size of the graphite is too small, anirreversible capacity increases. Thus, the average particle size of thegraphite is preferably 5 μm or more, more preferably 12 μm or more,particularly preferably 18 μm or more. From the viewpoint of theincrease of the capacity of the negative electrode, the average particlesize of the graphite is 30 μm or less, more preferably 25 μm or less,particularly preferably 20 μm or less.

The negative electrode may be produced by the following method, forexample: The negative electrode active material and an optional a binderand/or other additives are mixed to prepare a negative electrodemixture, and the mixture is dispersed in a solvent to prepare a paste.Preferably, the binder is dissolved in a solvent prior to mixing withthe negative electrode active material, and then mixed with the negativeelectrode active material and so on. The paste containing the negativeelectrode mixture is applied to a negative electrode current collectormade of a copper foil or the like, and then dried to form a negativeelectrode mixture layer. The layer is pressed to obtain a negativeelectrode. However, the method for producing the negative electrode isnot limited to the above-mentioned method, and may be any other method.

The density of the negative electrode mixture layer after pressing ispreferably 1.70 g/cm³ or more, more preferably 1.75 g/cm³ or more. Basedon the theoretical density of graphite, the upper limit of the densityof the negative electrode mixture layer formed using graphite is 2.1 to2.2 g/cm³. The density of the negative electrode mixture layer ispreferably 2.0 g/cm³ or less, more preferably 1.9 g/cm³ or less from theviewpoint of the affinity with the nonaqueous electrolyte. It ispreferable to press the negative electrode plural times since thenegative electrode can be uniformly pressed.

The binder used in the negative electrode is not particularly limited.For the increase of the content of the active material to increase thecapacity, the amount of the binder is preferably made as small aspossible. To this end, the binder is preferably a mixture of an aqueousresin which can be dissolved or dispersed, and a rubbery polymer, sincethe use of only a small amount of the aqueous resin can contribute tothe dispersion of the graphite and thus prevents the delamination of thenegative electrode mixture layer from the current collector caused bythe expansion and contraction of the electrode in the charge-dischargecycles.

Examples of the aqueous resins include cellulose resins such ascarboxymethylcellulose, and hydroxypropylcellulose, andpolyvinylpyrrolidone, polyepichlorohydrin, polyvinylpyridine, polyvinylalcohol, polyether resins such as polyethylene oxide and polyethyleneglycol, etc. Examples of the rubbery polymers include latex, butylrubber, fluororubber, styrene-butadiene rubber, nitrile-butadienecopolymer rubber, ethylene-propylene-diene copolymer (EPDM),polybutadiene, etc. From the viewpoint of the dispersibility of thegraphite particles and the prevention of delamination of the layer, itis preferable to use a cellulose ether compound such ascarboxymethylcellulose together with a butadiene copolymer rubber suchas a styrene-butadiene rubber. It is particularly preferable to usecarboxymethylcellulose together with a butadiene copolymer rubber suchas a styrene-butadiene copolymer rubber or a nitrile-butadiene-copolymerrubber. The cellulose ether compound such as carboxymethylcellulosemainly has a thickening effect on the paste containing the negativeelectrode mixture, while the rubbery polymer such as thestyrene-butadiene copolymer rubber has a binding effect on the negativeelectrode mixture. When the cellulose ether compound such ascarboxymethylcellulose and the rubbery polymer such as thestyrene-butadiene copolymer rubber are used in combination, the weightof the former to the latter is preferably from 1:1 to 1:15.

The thickness of the negative electrode mixture layer is preferably from40 to 200 μm. The thickness of the current collector used in thenegative electrode is preferably from 5 to 30 μm.

In the negative electrode mixture layer, the content of the binder orbinders is preferably 1.5% by weight or more, more preferably 1.8% byweight or more, particularly preferably 2.0% by weight or more of thelayer, while it is preferably less than 5% by weight, less than 3% byweight, less than 2.5% by weight. When the amount of the binder in thenegative electrode mixture layer is too large, the discharge capacity ofthe battery may decrease. When the amount is too small, the adhesionbetween the particles decreases. The content of the negative electrodeactive material in the negative electrode mixture layer is preferablymore than 95% by weight and 98.5% by weight or less.

In the present invention, a separator used in the present inventionpreferably has a thickness of 5 μm or more, more preferably 10 μm ormore, particularly preferably 12 μm or more, while it is preferably lessthan 25 μm, more preferably less than 20 μm, particularly preferablyless than 18 μm, from the viewpoint of imparting the directionality ofthe tensile strength to the separator, keeping good insulatingproperties and reducing the thermal shrinkage of the separator. The gaspermeability of the separator is preferably 500 second/100-mL or less,more preferably 300 second/100-mL or less, particularly preferably 120second/100-mL or less. As the gas permeability of the separator issmaller, the load characteristic is made better but an insideshort-circuit is more easily caused. Thus, the gas permeability ispreferably 50 second/100-mL or more. Here, a gas permeability ismeasured according to JIS P8117. As the thermal shrinkage factor of theseparator in the transverse direction (TD) is smaller, an insideshort-circuit is less easily caused when the temperature of the batteryrises. Thus, the thermal shrinkage factor in TD of the separator is assmall as possible. The thermal shrinkage factor in TD is preferably 10%or less, more preferably 5% or less. In order to restrain the thermalshrinkage of the separator, it is preferable to thermally treat theseparator in advance at a temperature of about 100 to 125° C. Theseparator having such a thermal shrinkage factor is preferably combinedwith the positive electrode materials according to the present inventionto fabricate a battery, since the behaviors of the battery at hightemperature become stable.

The thermal shrinkage factor in TD of the separator means the shrinkagefactor of a portion thereof that most largely shrinks in TD when theseparator having a size of 30 mm square is allowed to stand at 105° C.for 8 hours.

With regard to the strength of the separator, a tensile strength in themachine direction (MD) is preferably 6.8×10⁷ N/m² or more, morepreferably 9.8×10⁷ N/m or more. The tensile strength in TD is preferablysmaller than that in MD. The ratio of the tensile strength in TD to thatin MD (tensile strength in TD/tensile strength in MD) is preferably 0.95or less, more preferably 0.9 or less, while it is preferably 0.1 ormore. The transverse direction means a direction perpendicular to thedirection in which the film resin for the production of the separator iswound up, that is, the machine direction.

The puncture strength of the separator is preferably 2.0 N or more, morepreferably 2.5 N or more. As this value is higher, the battery is lesseasily short-circuited. Usually, however, the upper limit thereof issubstantially determined by the material of the separator. In the caseof a separator made of polyethylene, the upper limit of the puncturestrength is about 10 N. Here, a puncture strength is measured by cuttinga sample piece of 50 mm×50 mm from a separator, clamping the samplepiece with jigs at the edges of 5 mm, puncturing the sample piece with aneedle having a tip end with a radius of 0.5 mm at a rate of 2 mm/sec.,and measuring a maximum load before the puncture of the sample piece asa puncture strength.

When a conventional nonaqueous secondary battery is charged at a highpositive electrode voltage of 4.35 V or higher with reference to thepotential of lithium and is discharged to a final voltage higher than3.2 V, the crystalline structure of the positive electrode activematerial decays to decrease the capacity or to induce heating of thebattery due to the deterioration of the thermal stability. Thus, thebattery may not be practically used. When a positive electrode activematerial to which a different element such as Mg or Ti is added is used,the decrease of the safety or of the capacity over charge-dischargecycles can be suppressed, but the degree of suppression is notsufficient. Moreover, the filling of the positive electrode isinsufficient so that the battery easily expands.

In contrast, the battery of the present invention having the structureexplained above is a nonaqueous secondary battery which improves thecapacity, the charge-discharge cycle characteristic, the safety and thesuppression of expansion of the battery (storage characteristics) .These advantageous effects can be attained at a usual charging volt (abattery voltage of 4.2 V) . Furthermore, when the positive electrode ischarged up to a high voltage of 4.35 V with reference to the potentialof lithium (i.e., a battery voltage of 4.25 V) and then the discharge ofthe battery is terminated at a high voltage, that is, a battery voltageof 3.2 V or higher, the crystalline structures of the positive electrodeactive materials are very stable so that the decrease of the capacity orthermal stability is prevented.

Moreover, the positive electrode active material of any conventionalnonaqueous secondary battery generates a low average voltage. Therefore,when a charge-discharge cycle test is repeated under a condition thatthe discharge final voltage of a unit cell is 4.35 V or higher withreference to the potential of lithium, the positive electrode is dopedor dedoped with a large amount of Li ions. This situation is analogousto a case where the battery is subjected to a charge-discharge cycletest under an overcharge condition. Under such a severe condition, anyconventional positive electrode active material cannot maintain itscrystalline structure so as to cause disadvantages such that the thermalstability declines or the charge-discharge cycle life is shortened. Tothe contrary, the use of the positive electrode active materialaccording to the battery of the present invention can overcome suchdisadvantages of the conventional positive electrode active material.Thus, the present invention provides a nonaqueous secondary batterywhich can be reversibly charged and discharged even at a high voltage,such as a voltage of 4.35 to 4.6 V with reference to the potential oflithium, when the battery is fully charged.

Here, the “fully charged” means that a battery is charged by charging itat a constant current of 0.2C to a specific voltage and then charging itat a constant voltage at the specific voltage until the total chargingtime of the constant current charging and the constant voltage chargingreaches 8 hours. When the battery of the present invention has agraphite negative electrode (i.e. a negative electrode containinggraphite as a negative electrode active material) which has a voltage of0.1 V with reference to the lithium potential when the battery is fullycharged, the charging of the battery to a battery voltage of 4.45 V orhigher is assumed as a charging of the battery in which the voltage ofthe positive electrode is substantially 4.35 V or higher.

The nonaqueous secondary battery of the present invention hascharacteristics including a high voltage, a high capacity and a highsafety. By making use of such characteristics, the nonaqueous secondarybattery of the present invention can be used as a power source of anotebook personal computer, a stylus-operated personal computer, apocket personal computer, a notebook word processor, a pocket wordprocessor, an electronic book player, a cellular phone, a codelesshandset, a pager, a portable terminal, a portable copier, an electricalnotebook, an electronic calculator, a liquid crystal television set, anelectric shaver, an electric power tool, an electronic translatingmachine, an automobile telephone, a transceiver, a voice input device, amemory card, a backup power source, a tape recorder, a radio, aheadphone stereo, a handy printer, a handy cleaner, a portable CDplayer, a MD player, a portable digital audio player, a video movie, anavigation system, a refrigerator, an air conditioner, a television, astereo, a water heater, a microwave oven, a dishwasher, a washingmachine, a drying machine, a game equipment, a lighting equipment, atoy, a sensor equipment, a load conditioner, a medical machine, anautomobile, an electric vehicle, a golf cart, an electrically-poweredcart, a security system, a power storing system, or the like. Thebattery can be used not only for the consumer applications but also foraerospace applications. The capacity-increasing effect of the presentinvention is enhanced, in particular, in small-sized portable devices.Thus, the battery of the present invention is used in a portable devicedesirably having a weight of 3 kg or less, more desirably 1 kg or less.The lower limit of the weight of the portable device is not particularlylimited. However, the lower limit is desirably a value equal to theweight of the battery, for example, 10 g or more in order to attain theadvantageous effects to some degree.

The present invention will be described in detail with reference to thefollowing Examples; however, the Examples do not limit the scope of thepresent invention. Thus, modifications of the examples are encompassedby the scope of the present invention as long as the modifications donot depart from the subject matter of the present invention, which hasbeen described above or will be described hereinafter.

EXAMPLE 1

Production of Positive Electrode

The lithium-containing positive electrode materials,LiCo₀.₉₉₈Mg_(0.0008)Ti_(0.0004)Al_(0.0008)O₂ (average particle size: 12μm) as a positive electrode active material (A), andLiCo_(0.994)Mg_(0.0024)Ti_(0.0012)Al_(0.0024)O₂ (average particle size:5 μm) as a positive electrode active material (B) at a weight ratio of65:35 were mixed. Then, 97.3 parts by weight of the mixture and 1 .5parts by weight of a carbonaceous material as an electric conductive aidwere charged in a volumetric feeder as a device for supplying powder. Anamount of a 10 wt. % solution of polyvinylidene fluoride (PVDF) inN-methyl-2-pyrrolidone (NMP) to be supplied to the feeder was adjustedto control a solid content in the mixture constantly at 94% by weightduring kneading. While the amount of the mixed materials supplied in aunit time was controlled to a predetermined amount, the materials weresupplied in a biaxial kneading extruder and then kneaded. In this way, apaste containing the positive electrode mixture was prepared.

Separately, the positive electrode active materials (A) and (B) weredissolved in aqua regia and the ratio of the elements contained in thematerials (A) and (B) was measured by the ICP atomic emissionspectroscopy, the results of which confirmed that they had the aboveelementary compositions.

The resultant paste was charged in a planetary mixer, and then a 10 wt.% solution of PVDF in NMP, and NMP were added to dilute the paste,thereby adjusting the viscosity of the paste at a level sufficient forapplication. This diluted paste containing the positive electrode activematerial mixture was passed through a 70-mesh net to remove largesubstances. Thereafter, the paste was uniformly applied to both surfacesof a positive electrode current collector made of an aluminum foil witha thickness of 15 μm, and then dried to form film-form positiveelectrode mixture layers. In the dried positive electrode mixturelayers, the weight ratio of the positive electrode active material/theelectric conduction aiding agent/PVDF was 97.3:1.5:1.2. Thereafter, theresultant sheet was pressed and cut out in a predetermined size. To thecut piece, a lead member made of aluminum was welded to form asheet-form positive electrode. The density of the pressed positiveelectrode mixture layers (the density of the positive electrode) was3.86 g/cm³. The thickness of the positive electrode mixture layers (thetotal thickness of the layers on both the surfaces, i.e., the thicknessobtained by subtracting the thickness of the aluminum foil layer of thepositive electrode current collector from the total thickness of thepositive electrode) was 135 μm.

The particle size distribution of the mixture of the positive electrodeactive materials (A) and (B) was measured by a MICROTRAC particle sizeanalyzer (HRA 9320 available from NIKKISO Co., Ltd.). In the particlesize distribution curve, two peaks were found at an average particlesize of about 5 μm and 12 μm, respectively. d_(p) was larger than d_(m)and d_(p)/d_(M) was 1.4.

In the positive electrode active material (A), the amount of Mg was0.08% by mole, that of Ti was 0.04% by mole, and that of Al was 0.08% bymole, each based on the amount of Co. An electron prove X-raymicroanalyzer (EMPA 1600 manufactured by Shimadzu Corporation) was usedto measure the concentration of the metal element M² in cross sectionsof the particles. As a result, no difference in the concentration ofeach of Mg, Ti and Al was observed between the surface portion and thecore portion.

In the positive electrode active material (B), the amount of Mg was0.24% by mole, that of Ti was 0.12% by mole, and that of Al was 0.24% bymole, each based on the amount of Co. The concentration of the metalelement M² in the cross sections of the particles was measured in thesame manner as in the case of the positive electrode active material(A). As a result, no difference in the concentration of each of Mg, Tiand Al was observed between the surface portion and the core portion.

With regard to the contents of the metal elements M², the molar contentsof Mg, Ti and Al in the positive electrode active material (B) were 3times larger, 3 times larger and 3 times larger, respectively, thanthose in the positive electrode active material (A).

Production of Negative Electrode

As negative electrode active materials, 70 parts by weight of a graphitetype carbonaceous material (a) (purity: 99.9% or more, average particlesize: 18 μm, d₀₀₂: 0.3356 nm, size of the crystallite in the c axisdirection (Lc): 100 nm, R value of the Raman spectrum: 0.18) and 30parts by weight of a graphite type carbonaceous material (b) (purity:99.9% or more, average particle size: 21 μm, d₀₀₂: 0.3363 nm, size ofthe crystallite in the c axis direction (Lc): 60 nm, R value of theRaman spectrum: 0.11) were mixed. Then, 98 parts by weight of thegraphite mixture, 1 part by weight of carboxymethylcellulose and 1 partby weight of a styrene-butadiene rubber were mixed in the presence ofwater to prepare a paste containing negative electrode mixture. Thispaste was uniformly applied to both surfaces of a negative electrodecurrent collector made of a strip-form copper foil having a thickness of10 μm, and then dried to form negative electrode mixture layers. Theresultant sheet was pressed with a roller until the density of thenegative electrode mixture layers became 1.75 g/cm³. The resultant sheetwas then cut out in a predetermined size. Thereafter, a lead member madeof nickel was welded to the cut piece to form a sheet-form negativeelectrode.

Preparation of Nonaqueous Electrolytic Solution

An amount of LiPF₆ was dissolved in a mixed solvent of methylethylcarbonate, diethyl carbonate and ethylene carbonate mixed at a volumeratio of 1:3:2 to attain a concentration of 1.4 mol/L. To this solution,0.2% by weight of succinonitrile and 3% by weight of vinylene carbonatewere added to prepare a nonaqueous electrolytic solution.

Production of Nonaqueous Secondary Battery

The positive electrode and the negative electrode were spirally woundwith interposing, therebetween, a separator made of a microporouspolyethylene film (porosity: 53%, tensile strength in MD: 2.1×10⁸ N/m²,tensile strength in TD: 0.28×10⁸ N/m², thickness: 16 μm, gaspermeability: 80 seconds/100-mL, thermal shrinkage factor after beingkept at 105° C. for 8 hours: 3%, puncture strength: 3.5 N (360 g)), toform an electrode body having a spiral structure. Thereafter, theelectrode body was pressed to form a flat-shaped electrode body andinserted into a box-shaped battery case made of an aluminum alloy. Thepositive and negative lead members were welded and a cover plate waslaser welded to the edge portion of an opening of the battery case.Then, the nonaqueous electrolytic solution prepared in the above waspoured into the battery case through an inlet made in the cover plate.The nonaqueous electrolytic solution was sufficiently infiltrated intothe separator and the like. Thereafter, the battery was partiallycharged, and gas generated during the partial charging was discharged.Then, the inlet was sealed up to make the battery airtight. Thereafter,the battery was charged and aged to yield a rectangular nonaqueoussecondary battery having a structure as shown in FIGS. 1A and 1B and anexternal appearance as shown in FIG. 2, and a width of 34.0 mm, athickness of 4.0 mm, and a height of 50.0 mm.

Here, the battery shown in FIGS. 1A, 1B and 2 will be explained. Thepositive electrode 1 and the negative electrode 2 are spirally woundwith interposing the separator 3 therebetween, as described above, andthe spirally wound electrode body is pressed in a flat form to form theelectrode laminate 6 having a flat spiral structure. The laminate 6together with a nonaqueous electrolytic solution is contained in thebox-shaped battery case 4. For simplicity, in FIG. 1, metal foils ascurrent collectors used to form the positive electrode 1 and thenegative electrode 2, and the electrolytic solution are not depicted.

The battery case 4 is made of an aluminum alloy, and constitutes a mainpart of the exterior package of the battery. This battery case 4 alsofunctions as a positive electrode terminal. The insulator 5 made of apolytetrafluoroethylene sheet is arranged on the inside bottom of thebattery case 4. The positive electrode lead member 7 and the negativeelectrode lead member 8 connected to one end of the positive electrode 1and that of the negative electrode 2, respectively, are taken out fromthe electrode laminate 6 having the flat spiral structure. The terminal11 made of stainless steel is attached to the cover plate 9 made ofaluminum for closing the opening of the battery case 4 with interposingthe insulation packing 10 made of polypropylene therebetween. The leadplate 13 made of stainless steel is attached to this terminal 11 withinterposing the insulator 12 therebetween.

The cover plate 9 is inserted into the opening of the battery case 4,and their joining portions are welded to each other, thereby closing theopening of the battery case 4 to make the interior of the batteryairtight. In the battery shown in FIGS. 2A and 2B, the inlet 14 forpouring the electrolytic solution is made in the cover plate 9, and theinlet 14 is welded and sealed up by, for example, laser welding, withinserting a sealing member (not shown). In this way, the air-tightnessof the battery is kept. Accordingly, in the case of the battery shown inFIGS. 2A, 2B and 3, the electrolytic solution pouring inlet 14 isactually composed of the inlet 14 and the sealing member, but the inlet14 is illustrated as such without a sealing member in order to make thefigure simple. The explosion-proof vent 15 is made in the cover plate 9.

In the battery 1 of Example 1, the positive electrode lead member 7 isdirectly welded to the cover plate 9, whereby the combination of thebattery case 4 and the cover plate 9 functions as a positive electrodeterminal. The negative electrode lead member 9 is welded to the leadplate 13, and the negative electrode lead member 8 and the terminal 11are made electrically conductive through the lead plate 13, whereby theterminal 11 functions as a negative electrode terminal. However, thefunctions of the positive and negative electrodes may be reversed inaccordance with the material of the battery case 4, etc.

FIG. 2 is a perspective view schematically illustrating the externalappearance of the battery shown in FIGS. 1A and 1B. FIG. 2 shows thatthe above-mentioned battery is a rectangular battery. Thus, FIG. 2schematically shows the battery, and depicts the specific elements outof the constituting elements of the battery.

EXAMPLE 2

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that glutaronitrile was used in place ofsuccinonitrile.

EXAMPLE 3

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that adiponitrile was used in place of succinonitrile.

EXAMPLE 4

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the amount of succinonitrile was changed to 0.5%by weight.

EXAMPLE 5

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the amount of succinonitrile was changed to 1.0%by weight.

EXAMPLE 6

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the positive electrode active material (A) waschanged to LiCo_(0.9988)Mg_(0.0008)Ti_(0.0004)O₂ (average particle size:12 μm) , and the positive electrode active material (B) was changed toLiCo_(0.9964)Mg_(0.0024)Ti_(0.0012)O₂ (average particle size: 5 μm). Thedensity of the positive electrode mixture layers (positive electrode)after pressing was 3.79 g/cm^(3.) With regard to the contents of themetal elements M², the molar contents of Mg and Ti in the positiveelectrode active material (B) were 3 times larger and 3 times larger,respectively, than those in the positive electrode active material (A).

EXAMPLE 7

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the weight ratio of the positive electrode activematerial (A) to the positive electrode active material (B) was changedto 90:10. The density of the positive electrode mixture layers (positiveelectrode) after pressing was 3.75 g/cm³.

EXAMPLE 8

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that a mixture of the positive electrode activematerial (A) consisting ofLiCo_(0.998)Mg_(0.0008)Ti_(0.0004)Al_(0.0008)O₂ (average particle size:12 μm), and the positive electrode active material (B) consisting ofLiCo_(0.994)Mg_(0.0024)Ti_(0.0012)Al_(0.0024)O₂ (average particle size:5 μm) in a weight ratio of 50:50 was used as a positive electrode activematerial. The density of the positive electrode mixture layers (positiveelectrode) after pressing was 3.76 g/cm³. With regard to the contents ofthe metal elements M², the molar contents of Mg, Ti and Al in thepositive electrode active material (B) were 3 times larger, 3 timeslarger and 3 times larger, respectively, than those in the positiveelectrode active material (A).

EXAMPLE 9

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the positive electrode active material (A) waschanged to LiCo_(0.998)Mg_(0.0008)Ti_(0.0004)Sn_(0.0008)O₂ (averageparticle size: 14 μm), and the positive electrode active material (B)was changed to LiCo_(0.994)Mg_(0.0024)Ti_(0.00i2)Sn_(0.0024)O₂ (averageparticle size: 6 μm) . The density of the positive electrode mixturelayers (positive electrode) after pressing was 3.76 g/cm³. With regardto the contents of the metal elements M², the molar contents of Mg, Tiand Sn in the positive electrode active material (B) were 3 timeslarger, 3 times larger and 3 times larger, respectively, than those inthe positive electrode active material (A).

EXAMPLE 10

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the positive electrode active material (A) waschanged to LiCo_(0.998)Mg_(0.0008)Zr_(0.0004)Al_(0.0008)O₂ (averageparticle size: 13 μm), and the positive electrode active material (B)was changed to LiCoo.994Mg0.0024Zr0.0012A10.0024O2 (average particlesize: 6 μm) . The density of the positive electrode mixture layers(positive electrode) after pressing was 3.8 g/cm³. With regard to thecontents of the metal elements M², the molar contents of Mg, Zr and Alin the positive electrode active material (B) were 3 times larger, 3times larger and 3 times larger, respectively, than those in thepositive electrode active material (A).

EXAMPLE 11

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the positive electrode active material (A) waschanged to LiCo_(0.998)Mg_(0.0008)Ge_(0.0004)Al_(0.0008)O₂ (averageparticle size: 12 μm), and the positive electrode active material (B)was changed to LiCo_(0.994)Mg_(0.0024)Ge_(0.0012)Al_(0.0024)O₂ (averageparticle size: 6 μm) . The density of the positive electrode mixturelayers (positive electrode) after pressing was 3.79 g/cm³. With regardto the contents of the metal elements M², the molar contents of Mg, Geand Al in the positive electrode active material (B) were 3 timeslarger, 3 times larger and 3 times larger, respectively, than those inthe positive electrode active material (A).

EXAMPLE 12

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that the positive electrode active material (A) waschanged toLiCo_(0.334)Ni_(0.33)Mn_(0.33)Mg_(0.0024)Ti_(0.0012)Al_(0.0024)O₂(average particle size: 5 μm). The density of the positive electrodemixture layers (positive electrode) after pressing was 3.72 g/cm³. Withregard to the contents of the metal elements M², the molar contents ofMg, Ti and Al in the positive electrode active material (B) were 3 timeslarger, 3 times larger and 3 times larger, respectively, than those inthe positive electrode active material (A).

COMPARATIVE EXAMPLE 1

A nonaqueous secondary battery was fabricated in the same manner as inExample 1 except that onlyLiCo_(0.998)Mg_(0.0008)Ti_(0.0004)Al_(0.0008)O₂ (average particle size:12 μm) was used as a positive electrode active material and nosuccinonitrile was added to the nonaqueous electrolyte. The density ofthe positive electrode mixture layers (positive electrode) afterpressing was 3.7 g/cm³. This is a comparative example using only apositive electrode active material having a large particle size, thatis, the positive electrode active material (A), among the positiveelectrode active materials used in Example 1 and further no compoundhaving at least two nitrile compound was used.

With the nonaqueous secondary batteries fabricated in Examples 1-12 andComparative Example 1, the following properties were evaluated:

Discharge Capacity After Charge-Discharge Cycles

Each of the batteries fabricated in Examples and Comparative Example wasdischarged to 3.0 V at 1 CmA at a room temperature, and charged to 4.2 Vat a constant current of 1 C and then at a constant voltage of 4.2 Vuntil the total charge time reached 2.5 hours . Thereafter, the batterywas discharged at 0.2 CmA down to 3.0 V. Thereby, a discharge capacityof the battery was measured. Then, the above charge-discharge cycle wasrepeated five times, and the discharge capacity after the fifth cyclewas used to evaluate the discharge capacity after the charge-dischargecycles. The results are shown in Table 1. In Table 1, the dischargecapacity after the charge-discharge cycles obtained with each battery isshown as a relative value in relation to the discharge capacity of thebattery of Comparative Example 1 after the charge-discharge cycles,which is “100”.

Evaluation of Storage Characteristics

Each of the batteries fabricated in Examples and Comparative Example wascharged to 4.2 V at a constant current of 1 C and then at a constantvoltage of 4.2 V until the total charge time reached 2.5 hours, followedby discharging at 1 CmA down to 3.0 V. Thereafter, the battery was againcharged to 4.2 V at a constant current of 1 C and then at a constantvoltage of 4.2 V until the total charge time reached 2.5 hours. Then,the thickness Tb of the battery (before storage) was measured. After themeasurement of the thickness, the battery was stored in a thermostaticchamber kept at 85° C. for 24 hours, removed from the chamber and keptstanding at room temperature for 4 hours. Again, the thickness Ta of thebattery (after storage) was measured. Using the thicknesses of thebattery before and after storage, a rate of change of the batterythickness was calculated according to the following equation:

Rate of change (%)=[(Tb−Ta)/Ta]×100

The results are shown in Table 1. TABLE 1

Example Discharge capacity after Rate of change of No. charge-dischargecycles battery thickness 1 102 14.8 2 102 13.2 3 103 11.6 4 101 12.6 599 9.8 6 103 15.3 7 102 15.6 8 103 15.2 9 102 15.3 10  103 15.4 11  10315.6 12  104 17.6 C. 1 100 25.0

As can be seen from the results in Table 1, the nonaqueous secondarybatteries of Examples 1-12 according to the present invention had thebetter discharge capacity, charge-discharge cycle characteristics andstorage characteristics than the nonaqueous secondary battery ofComparative Example 1.

1. A nonaqueous secondary battery comprising: a positive electrodehaving a positive electrode mixture layer, a negative electrode, and anonaqueous electrolyte, wherein the positive electrode contains, as anactive material, at least two lithium-containing transition metal oxideshaving different average particle sizes, a lithium-containing transitionmetal oxide having the smallest average particle size among said atleast two lithium-containing transition metal oxides having differentaverage particle sizes is a lithium-containing transition metal oxiderepresented by the formula (I):Li_(x)M¹ _(y)M² _(z)M³ _(v)O₂   (1) wherein M¹ represents at least onetransition metal element selected from Co, Ni and Mn, M² represents Mgand at least one metal element selected from the group consisting of Ti,Zr, Ge, Nb, Al and Sn, M³ represents an element other than Li, M¹ andM², and x, y, z and v are numbers satisfying the equations respectively:0.97≦x<1.02, 0.8≦y<1.02, 0.002≦z≦0.05, and 0≦v≦0.05, the positiveelectrode mixture layer has a density of at least 3.5 g/cm³, and thenonaqueous electrolyte contains succinonitrile.
 2. The nonaqueoussecondary battery according to claim 1, wherein said nonaqueouselectrolyte contains succinonitrile in an amount of 0.005 to 1% byweight based on the whole weight of the nonaqueous electrolyte.
 3. Thenonaqueous secondary battery according to claim 1 or 2, wherein, amongsaid at least two lithium-containing transition metal oxides havingdifferent average particle sizes, a lithium-containing transition metaloxide or lithium-containing transition metal oxides other than thelithium-containing transition metal oxide having the smallest averageparticle size is a lithium-containing transition metal oxide representedby the formula (2):Li_(a)M^(1′) _(b)M^(2′) _(c)M^(3′) _(d)O₂   (2) wherein M^(1′)represents at least one transition metal element selected from Co, Niand Mn, M^(2′) represents Mg and at least one metal element selectedfrom the group consisting of Ti, Zr, Ge, Nb, Al and Sn, M^(3′)represents an element other than Li, M^(1′) and M^(2′), and a, b, c andd are numbers satisfying the equations respectively: 0.97≦a<1.02,0.8≦b<1.02, 0≦c≦0.02, and 0≦d≦0.02.
 4. The nonaqueous secondary batteryaccording to claim 3, wherein, among said at least twolithium-containing transition metal oxides having different averageparticle sizes, a lithium-containing transition metal oxide orlithium-containing transition metal oxides other than thelithium-containing transition metal oxide having the smallest averageparticle size has an average particle size of 5 to 25 μm.
 5. Thenonaqueous secondary battery according to claim 1 or 2, wherein, amongsaid at least two lithium-containing transition metal oxides havingdifferent average particle sizes, the lithium-containing transitionmetal oxide having the smallest average particle size has an averageparticle size of 2 to 10 um.
 6. The nonaqueous secondary batteryaccording to claim 1 or 2, wherein said nonaqueous electrolyte furthercontains vinylene carbonate.